A variety of techniques are known for reducing the power consumption in computer systems. For example, the Advanced Configuration and Power Interface (ACPI) Specification (Rev. 2.0a, Mar. 31, 2002) sets forth information about how to reduce the dynamic power consumption of portable and other computer systems. With respect to processors used in computer systems, different processor power states (or C states) are defined in the ACPI Specification. Mobile computer systems typically include implementation of a variety of C states (e.g., C0, C1, C2, and C3) to save power.
In the C0 state, the processor is considered to be in an active state, executing code and performing useful work. In any of the non-C0 states, the processor may be considered to be idle. For example, in the C1 state, the processor may be able to respond to snoop transactions, and wake up very quickly (also referred to as having low latency exit). The processor may progressively cycle through the other lower states such as C2 and C3 as necessary to save even more power. C2 is a stop grant or mobile “Quick Start” state, where typically clocks are gated off to a large portion of the processor die. The processor is able to service snoops in the C2 state. The C2 state also supports a fairly low latency exit to allow software to quickly resume execution. A processor in the C2 state may not return to the C0 state as quickly as a processor in the C1 state. However, a processor in the C2 state may not consume as much power as a processor in the C1 state.
C3 is a deep sleep state, where clocks may be gated either internally or externally to the entire processor. In the C3 state, the processor is unable to snoop bus master transactions. A processor in the C3 state may have higher latency exit to return to the C0 state as compared to a processor in the C2 or C1 state.